1. Field of the Invention
The present invention relates to an electronic system comprising a plurality of units, devices or sets, which access a network by means of a multiple access protocol and also relates to a suitable method for multiple access to the network and to a computer program for performing the method for multiple access.
2. Description of the Related Art
Different multiple access protocols are known in the state of the art. A review of the different multiple access protocols is provided in “Computer Networks” by Andrew S. Tannenbaum, Prentice Hall, 1998, especially chapter 4.2, pp. 272 and following.
In the so-called ALOHA system the channel assignment occurs so that different users are uncoordinated during concurrent use of a single channel. Pure ALOHA and subdivided ALOHA should be distinguished from each other; in a pure ALOHA method there can be collisions between frames during multiple access. However because of the acknowledgement possibilities in broadcast technology a user can always establish, whether the frame to be transmitted would be destroyed, since the user hears the corresponding messages in the channel.
If the frame would be destroyed, the user waits for a predetermined time interval and then transmits it. Such systems, in which the user uses a common channel so that collisions can occur, are generally known as concurrent systems (contention systems).
In the subdivided ALOHA method the time is partitioned into individual intervals. Each interval corresponds to a frame. In this process in contrast to pure ALOHA the user does not immediately transmit as soon as a line flyback is detected. Instead of that the user waits until the next time slot. In so far as it is a matter of a slotted ALOHA method, subdivided ALOHA is a time-discrete variation of a pure ALOHA method.
Furthermore different methods for solving the problem of collisions are known in slotted ALOHA methods, e.g. the so-called exponential-backoff. A detailed explanation of this latter method is found in B. Walke, “Mobile Nets and their Protocols—Volume 1”, Teubner, Stuttgart, 1998.
No methods, which operate according to a multiple access protocol, are used in security networks for monitoring buildings and plants, e.g. for fire and break-in protection, or in other safety-critical networks. In this type of system a definite fixed time slot is assigned to each unit or set in the network. A suitable frame may also have a number of time slots, which correspond to the number of sets or units of the network.
Thus it is guaranteed that each set or unit of the network can access the network during each interrogation cycle. This however is disadvantageous because the bandwidth of the network must be high in order to achieve a short access time, since all the units must have a dedicated time slot in each time frame. The so-called latency time for access of a unit to the network thus becomes comparatively long when the bandwidth of the network is insufficient.